The invention relates to a process for producing sodium carbonate.
It relates more particularly to a process for producing sodium carbonate from an ore comprising sodium sesquicarbonate.
Trona ores, deposits of which exist particularly in the state of Wyoming in the United States, are used for the production of sodium carbonate. The usable matter in these ores is sodium sesquicarbonate, which is generally present in a quantity on the order of 80 to 95% by weight.
Various processes for enriching these ores and for extracting sodium carbonate from them have been proposed, which could be improved, particularly in the glass industry. In one known enrichment process (International Application WO 94/27725), the ore is subjected to several successive physical operations, including a grinding, a particle size fractionation into a fine size fraction and a coarse size fraction, and a densimetric, electrostatic or magnetic classification of the fine size fraction. In this known process, the purpose of grinding and the particle size classification is to enrich the ore (the fine size fraction being enriched with sodium sesquicarbonate, to the detriment of the coarse size fraction). In a variant of this known process, it is recommended that prior to the densimetric classification, the ore be subjected to a calcination in order to convert the sodium sesquicarbonate into anhydrous sodium carbonate. In this variant of the known process, the purpose of the calcination is essentially to facilitate the densimetric classification by increasing the difference between the specific gravity of the usable matter of the ore and that of the barren matter forming the gangue. It can be performed either before the grinding or after the particle size fractionation.
The known process just described has the disadvantage of generating large volumes of waste, containing non-negligible quantities of usable matter. Moreover, the quantity of usable matter lost in the waste increases with the degree of purity sought for the sodium carbonate to be produced. For this reason, this known process does not make it possible to produce sodium carbonate of high purity economically. The degree of purity of the sodium carbonate obtained rarely exceeds 98% by weight.
The process described in U.S. Pat. No. 3,425,795 eliminates this drawback. According to this known process, after having been finely ground, then calcined to convert the sodium sesquicarbonate into anhydrous sodium carbonate, the ore is dispersed in an aqueous solution, saturated with sodium carbonate monohydrate, the resulting aqueous suspension is subjected to an aging at a temperature of 65 to 108xc2x0 C. (preferably 92 to 97xc2x0 C.), and at the end of the aging, a mixture comprising sodium carbonate monohydrate crystals and insoluble compounds is collected, and the sodium carbonate monohydrate crystals are extracted from the mixture by means of a particle size fractionation. The sodium carbonate monohydrate crystals can then be heated above the temperature of 108xc2x0 C. in order to convert the sodium carbonate monohydrate into anhydrous sodium carbonate. The known process just described makes it possible to obtain sodium carbonate with a purity higher than 99.5% by weight.
An improved process has now been found, which makes it possible to increase the purity of the sodium carbonate even further.
Consequently, the invention relates to a process for producing sodium carbonate according to which a sodium sesquicarbonate ore is successively calcined and dispersed in an aqueous solution substantially saturated with hydrated sodium carbonate, and the resulting aqueous suspension is subjected to an aging, at the end of which an aqueous mixture of a powder comprising hydrated sodium carbonate crystals is collected, the process being characterized in that during the aging, a fine size fraction is extracted from the aqueous suspension and removed.
In the process according to the invention, the sodium sesquicarbonate is a mineral with the general formula Na2CO3.NaHCO3.2H2O. The source of the sodium sesquicarbonate ore is not critical. It could be, for example, a trona ore originating from the state of Wyoming in the United States, normally containing from 80 to 95% sodium sesquicarbonate by weight.
In the process according to the invention, the ore is calcined. Calcination is an operation that is well known in the art. It consists of subjecting the ore to a heat treatment under controlled conditions in order to break down the sodium sesquicarbonate and form anhydrous sodium carbonate. The heat treatment generally comprises a heating to a temperature higher than 100xc2x0 C., preferably at least equal to 120xc2x0 C., for example between 125 and 200xc2x0 C. The technique used to perform the calcination of the ore is not critical. Advantageously, the technique described in the document WO 94/27725 may be used.
In the process according to the invention, the calcined ore is dispersed in an aqueous solution that is substantially saturated with hydrated sodium carbonate, and the aqueous suspension thus formed is subjected to an aging. During the aging, the anhydrous sodium carbonate of the calcined ore recrystallizes into hydrated sodium carbonate. The aging in this case must be performed under physicochemical conditions in which the hydrated sodium carbonate is stable, which conditions can be easily determined by one skilled in the art. The hydrated sodium carbonate that crystallizes can be sodium carbonate monohydrate, sodium carbonate heptahydrate or sodium carbonate decahydrate, depending on the physicochemical conditions used in the aging. For example, in the case where the hydrated sodium carbonate is sodium carbonate monohydrate (with the general formula Na2CO3.H2O), the aging is performed at a temperature that is lower than the temperature of the transition of the sodium carbonate monohydrate into anhydrous sodium carbonate, this transition temperature being in the neighborhood of 108xc2x0 C. at the normal atmospheric pressure. Temperatures between about 35 and 108xc2x0 C. are generally suitable, and temperatures from 80 to 100xc2x0 C. are particularly advantageous. Preferably, the aging should also be performed under suitable pressure and temperature conditions, in order to prevent an evaporation of the hydrated sodium carbonate solution.
The process according to the invention is based on a modification of the diameter of the sodium carbonate particles, by recrystallization into the hydrated state during the aging. More particularly, the process uses operating conditions for which the average diameter of the hydrated sodium carbonate crystals that are formed during the aging stage is larger than the diameter of the particles of the calcined ore. The average diameter of the hydrated sodium carbonate crystals is defined by the mathematical relation
d=xcexa3nidi/xcexa3ni
in which d designates the average diameter and ni designates the gravimetric frequency of the crystals of diameter di. The diameters di are, for example, measured by screening in accordance with the AFNOR standard. During the aging, the conditions created preferably promote the production of hydrated sodium carbonate crystals of coarse size and uniform morphology, having a particle size distribution that is not very wideranging. The process according to the invention normally requires an appropriate grinding of the calcined ore, possibly followed by a screening, in order to accentuate the difference between the size of the particles of the calcined ore and the size of the hydrated sodium carbonate crystals formed during the aging.
At the end of the aging, a mixture of a powder in suspension in an aqueous solution saturated with hydrated sodium carbonate is collected. The powder is essentially composed of hydrated sodium carbonate crystals and of normally insoluble materials of the gangue of the ore. The powder is subjected to a particle size fractionation. The purpose of the particle size fractionation is to divide the powder into at least two parts with two distinct particle sizes, one of which is enriched with hydrated sodium carbonate as compared to the other. In the process according to the invention, it is essential during the particle size fractionation of the mixture to maintain physicochemical conditions (especially a pressure and a temperature) that prevent a decomposition of the hydrated sodium carbonate crystals. The particle size fractionation can be performed by any appropriate means. A first means consists of extracting the powder from the mixture, drying it and then subjecting it to a screening. A second means, which is preferred, consists of subjecting the powder of the mixture to an elutriation. Elutriation is a well-known technique for particle size analysis (Particle Size Measurement, Terence Allen, Chapman and Hall, London 1974, pages 250-263). The elutriation is preferably performed wet. It is advantageous to select an elutriation of the levigation type.
According to the invention, during the aging, a fine particle size class is extracted from the aqueous suspension and is removed. The term fine particle size class is intended to designate a set of solid particles of the aqueous suspension whose diameter is smaller than the average diameter of the hydrated sodium carbonate crystals obtained at the end of the aging.
The extraction of the fine particle size class from the aqueous suspension can be performed in any appropriate particle size classifier. According to one particular embodiment of the process according to the invention, a fraction of the aqueous suspension in the process of being aged is periodically or continuously sampled, said aqueous suspension fraction is processed in a particle size classifier in which the solid part of said aqueous fraction is divided into a fine particle size class and a coarse particle size class, the fine particle size class is removed, and said aqueous suspension fraction (freed of its fine particle size class) is recycled into the aqueous suspension in the process of being aged. The particle size classifier can be, for example, the type that screens through standardized screens. It is preferred to use a particle size separator that uses elutriation, a wet elutriation of the levigation type being preferred. In the particular embodiment of the process according to the invention just described, the purity of the sodium carbonate obtained at the end of the process increases in proportion to the relative size of the fraction of the aqueous suspension that is extracted and subjected to the particle size classification. In practice, it is advantageous for this fraction to be at least equal to 1:1 (preferably 5:1) per 1 kg of calcined ore used, and not to exceed 500:1 (preferably 25:1) per 1 kg of said calcined ore. The purity of the sodium carbonate also depends on the cutoff diameter of the two particle size classes. By definition, the cutoff diameter is the aperture diameter of the standardized particle size measuring screen (for example according to the AFNOR standard), which the entire fine particle size class passes through and which stops the entire coarse particle size class. The cutoff diameter must be smaller than the average diameter of the hydrated sodium carbonate crystals in the aqueous mixture obtained at the end of the aging. All else being equal, the larger the above-mentioned cutoff diameter, the purer the sodium carbonate obtained at the end of the process according to the invention. On the other hand, all else being equal, the volume of the fine particle size class (and consequently the loss of usable matter) increases with the cutoff diameter. The selection of the optimum cutoff diameter is therefore the result of a compromise between the search for a maximum purity of the sodium carbonate produced and for a minimum loss of usable matter from the ore.
In the process according to the invention, the sodium sesquicarbonate ore may possibly undergo a physical enriching pretreatment, before being dispersed in the aqueous solution of hydrated sodium carbonate. This physical enriching pretreatment may specifically include a densimetric, electrostatic and/or magnetic classification. It can be performed either before or after the calcination. It is preferable to perform it after the calcination. Densimetric, electrostatic and magnetic classifications are well known physical methods for enriching ores. Information about these three methods is specifically available in the document WO 94/27725.
At the end of the process according to the invention, the hydrated sodium carbonate is collected. It can be used as is. Preferably, it is converted into anhydrous sodium carbonate, by heating it to a temperature higher than the temperature of the transition of the sodium carbonate monohydrate into anhydrous sodium carbonate.
The process according to the invention has an especially recommended application to the production of sodium carbonate from trona ores, particularly ores comprising from 80 to 95% sodium sesquicarbonate by weight, deposits of which are found in the state of Wyoming in the United States.